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Compound Interest Calculator
Growth of savings with monthly contributions and daily, monthly or yearly compounding, with a year-by-year table.
Calculated in your browser – nothing you enter is sent or stored.
This compound interest calculator shows how an initial deposit and regular monthly contributions grow over time when interest earns interest. Choose daily, monthly, quarterly or annual compounding, see the split between your deposits and the interest earned, and follow the balance year by year in the chart and table. Add an inflation rate to see the result in today's dollars.
The Compound Interest Formula
For a single deposit, the future value is:
A = P × (1 + r/n)n·t
- A – the final amount
- P – the initial deposit (principal)
- r – the annual interest rate as a decimal (5% = 0.05)
- n – the number of times interest compounds per year (12 for monthly, 365 for daily)
- t – the number of years
Regular contributions add the future value of an annuity: PMT × [(1 + i)m − 1] ÷ i, where i is the monthly rate and m the number of months. The calculator works through every month, so it handles any mix of compounding frequency and deposit timing exactly.
Worked Example
$10,000 invested at 5% compounded monthly for 10 years grows to $16,470.09 – $6,470.09 of interest without adding another dollar. Add $200 a month and the balance reaches $47,526.55: $34,000 of deposits and $13,526.55 of interest.
How Compounding Frequency Changes the Result
More frequent compounding earns slightly more, because interest starts earning interest sooner. For $10,000 at 5% over 10 years:
| Compounding | Final balance | APY |
|---|---|---|
| Annually | $16,288.95 | 5.000% |
| Quarterly | $16,436.19 | 5.095% |
| Monthly | $16,470.09 | 5.116% |
| Daily | $16,486.65 | 5.127% |
The difference is small. The rate itself, how much you contribute and how long you leave the money alone matter far more. Banks advertise savings accounts and CDs by their APY (annual percentage yield), which already includes compounding – so when you compare accounts, compare APYs.
The Rule of 72
A quick way to estimate how long it takes money to double: divide 72 by the annual rate. At 6%, money doubles in about 72 ÷ 6 = 12 years; at 9%, in about 8 years. The calculator shows the exact doubling time for your rate.
Why Starting Early Matters
Time is the most powerful part of the formula. Someone who invests $300 a month from age 25 to 35 and then stops will usually have more at 65 than someone who invests $300 a month from 35 to 65 – at a 7% return, roughly $421,000 compared with $366,000 – despite contributing a third as much. The early money simply has 30 more years to compound.
What Rate Should I Use?
- High-yield savings and money market accounts: use the current APY offered by your bank.
- CDs and Treasury bills: use the fixed yield for the term.
- Stock index funds: the S&P 500 has averaged roughly 10% a year before inflation over long periods, but returns vary widely year to year. Many planners use 6% to 7% for long-term estimates.
Investment returns are not guaranteed, and taxes on interest and gains reduce growth outside tax-advantaged accounts such as a 401(k) or IRA. To see what inflation does to the buying power of your savings, use the inflation calculator, and for paying off debt instead, the mortgage calculator shows how extra payments save interest.
Sources
- Investor.gov (SEC) – Compound interest calculator and basics
- FDIC – National rates and rate caps for deposit accounts
- TreasuryDirect – Treasury bills, notes and bonds
Last reviewed: October 2026 by the ShrinkBee team. Results are estimates for planning and education, not financial, tax, legal or medical advice.
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